An electric double layer capacitor uses activated carbon as the material of the electrodes. The activated carbon has a large specific surface and is electrochemically inactive. The electrodes are combined with an electrolyte to provide a large electric double layer capacitance. The electric double layer capacitor makes use of this large electric double layer capacitance. Therefore, it is expected that such electric double layer capacitors will act as power sources of electric vehicles or other vehicles producing no exhaust gas in the future.
When a voltage is applied between the electrodes of an electric double layer capacitor, two serially connected electric double layers are formed and the capacitor is charged. However, if the voltage exceeds the decomposition voltage of the electrolyte, then an electrical current begins to flow between the electrodes.
Accordingly, the working voltage of an electric double layer capacitor depends on the decomposition voltage of the electrolyte being used. It is said that the dielectric strength is about 1.2 volts where the electrolyte is an aqueous solution and about 5 volts where the electrolyte is a nonaqueous solution. Presently, commercially available electric double layer capacitors have maximum working voltages of 0.5 to 2.8 volts per unit cell and capacitances of several farads. The internal resistances vary widely, from about 0.1 to 100 .OMEGA.. A recently published prototype of an electric double layer capacitor has a maximum working voltage of 2.5 volts, a capacitance of 240 farads and an internal resistance of about 0.1 .OMEGA..
When such a large capacity electric double layer capacitor is charged with its maximum permissible voltage to use the capacitor in power applications, the electrode material and the electrolyte are decomposed by electrolysis though the rate of decomposition is slow. As a result, the capacitance drops and the internal resistance rises slowly. Thus, the capacitor deteriorates. Finally, the capacitor can no longer be used. The decomposition rates of the electrolyte decreases sharply to an almost negligible level when the applied voltage to the capacitor is lowered below a certain voltage.
We have already proposed an example of a circuit for electrically charging an electric double layer capacitor in U.S. patent application Ser. No. 08/041,543.
The energy that can be stored in a capacitor is in proportion to the square of the charging voltage, i.e., CV.sup.2 /2, where C is the capacitance. Therefore, in order to increase the stored energy, the charging voltage should be made as high as possible.